Passenger seats, and particularly vehicle or aircraft passenger seats, are designed to ensure passenger safety for various loading conditions including, for example, hard landings and crash conditions. In some instances, regulatory and/or governmental requirements dictate that seats comply with occupant protection/head impact tests. The general intent is that the seat has a joint or mechanism designed to rotate the seatback forward during an impact, such that when a passenger from a row behind the seat moves forward (due to rapid deceleration of the vehicle or aircraft), the impact forces between the passenger's head and the seat can be reduced. Reducing head accelerations/decelerations and the related forces decrease the likelihood of head trauma and head injuries. However, the seats must also withstand typical non-crash load abuse, such as passengers leaning heavily against the seat, using the seat as a brace, and other non-crash loading conditions.
Many seatbacks are attached to a stable quadrant arm on each side of a seat frame. In order to meet head impact criteria testing, the seat is designed to distribute head impact loads from the seat back to shear pins on both sides of the seat back. The shear pins constrain rotation of the back relative to the quadrant arms until impact. At impact, the shear pins serve as a break over device, designed to fail during an impact event and to allow the back to rotate forward. This can reduce head accelerations. However, shear pins have strict limits on breakout force and timing because they must be strong enough to survive static loading and can only allow break over when impact loads exceed the ultimate load on both pins. The challenge is often that because the shear pins must withstand general abuse loads, they may be so strong as to require excessive acceleration in order to break/shear properly. However, if the strength of the frangible joint/shear pin is reduced, the seat may not be strong enough to withstand expected general abuse loads. For example, some of the current seat designs have problems during 10 degree impact events, when asymmetrical loading on the back requires high rigidity in the back structure to transfer sufficient loads to both shear pins. In general, a shear pin break over device necessitates a highly reinforced seatback structure that is rigid enough to predictably transmit loads to both quadrant arms, but also cushioned to reduce head accelerations on initial impact. In some cases, despite a rigid structure for a seat back, loads distributed to a quadrant arm on one side are significantly different than load distributed to a second quadrant arm of the second side of the seat. Such a structural arrangement can add weight to the seatback structure, and can require costly iterations of testing.
In certain situations, it may be desirable to design seats to efficiently, predictably, and repeatably rotate the seatback forward during crash or other loading conditions.